Hot melt adhesives are well known. Generally, hot-melt adhesives are applied by melting the adhesive composition and applying a coat of the molten adhesive layer on a substrate. The coated material is then cooled to harden the adhesive layer and is ready for storage. Among the hot-melt adhesives that have been found useful and economically important are remoistenable hot-melt adhesives and remoistenable pressure-sensitive hot-melt adhesives.
Water activatable adhesives applied from water-based media are commonly used as envelope-flap adhesives, postage-stamp adhesives, binding tapes, sealing tapes, diapers and the like. Now, remoistenable hot-melt adhesives can be used for similar applications. For such use, it is desirable that the hot-melt coating be capable of storage without blocking, i.e. adhering due to activating of the adhesive by combination of ambient humidity, temperature and contact pressure. In addition, the dried adhesive, when contacted with water, must be uniformly activated and capable of developing an adherent bond between the coated surface and an uncoated surface. The hot-melt characteristics of the formulations are also important. The adhesive should have good pot life, at least 30-40 hours at usual application temperatures of about 350.degree. F., as well as low viscosity characteristics at these temperatures, and the viscosity of the hot-melt should be substantially constant during the pot-life of the hot-melt.
As a result of previous work in this field, a water activatable adhesive which has excellent non-blocking and water activatable adhesive characteristics, and also excellent hot-melt characteristics has been found, as disclosed in U.S. Pat No. 4,325,851.
Pressure sensitive hot-melt adhesives are useful for the adhesive coating of labels, cloth patches and the like. A water soluble pressure-sensitive hot-melt adhesive composition with excellent adhesive properties has been disclosed in U.S. Pat. No. 4,331,576. Labels comprised of this water soluble pressure-sensitive hot-melt adhesive composition adhere permanently to substrates under normal conditions; on the other hand, when wet, such labels can be removed readily without damage to the substrate.
Non-pressure sensitive hot-melt adhesives have been used as the adhesive medium for multiline construction of disposable baby and adult diapers, sanitary napkins and hospital bed pads. In this construction the adhesive is applied in longitudinal, parallel or bead multi-lines to laminate a polyolefin film which forms the outer shell to tissues or non-woven substrates. It is often desirable to know if it is wet and thus a hot melt adhesive used in this capacity, and yet signaling the presence of water by a color change is invaluable.
An example of a non-pressure sensitive water based latex adhesive which when dry, signals the presence of water by a color change is taught in U.S. Pat. No. 4,231,370, issued to Mroz et al. According to this disclosure a flexible pH-change/color wetness indicator is coated on a surface portion of the product, which is visible through the cover member, and which retains sharp edge definition of the coated surface portion when wetted, for example, by urine. Such a coating includes a pH-change/color-change type material dispersed in a polymer latex matrix composed of styrene/2-ethylhexyl-acrylate copolymer, vinyl acetate/ethylene copolymer and polyvinyl acetate. To obtain a suitable pH, sufficient acid buffering means, such as phosphoric acid must be used; this is a harsh acid which could conceivably hurt a child.
It is a disadvantage of the broadly similar art, such as the Mroz et al composition, that the adhesive material can only be obtained by evaporation from a water-based latex composition. This means that equipment must be provided during the manufacture of the Mroz et al product which can release the water present. Also such problems as foaming, and proper wetting of the substrate must be overcome, all resulting in a more expensive manufacturing process of the Mroz et al formula. It is another disadvantage that any color change of the Mroz et al composition takes a very long time to take effect, e.g of the order of 3 minutes, as can be seen, for example, from FIGS. 3 and 4 of the Mroz et al patent.
Further, the time required for the color change in the Mroz et al composition is dependent on the thickness of the coating, as well as the pH of the wetting composition; the thicker the coating and the lower the pH, the slower the color change.
It is often desirable to know quickly, by visual inspection, whether a substrate has become wet. For example, it is desirable to know when a diaper has become set, but the wetness of a diaper which is plastic coated, or which is worn under a water proof panty, is not readily determined by visual inspection.